Most of the codes are portable to Gateshark since the reverse is possible.
BUT, complex code with a lot of data manipulation (data structure, encryption handling, etc...) can't be ported since you're limited to the operations given by the Gateshark system.
Also, note that as there are a few codes which work under Gateshark but not on NTR, the opposite might be possible as well.
Last thing: it's not possible to convert a plugin file (.plg) into a Gateshark code.
You need the source code of the plugin to convert it.